1. Technical Field
The present invention relates to a method and an apparatus for controlling reactant ion concentration within a precipitation vessel to optimize the reactions occurring therein, and more particularly, the invention comprises such a method and apparatus for control of halide ion concentration in a reactor during the precipitation and growth of silver halide grains.
2. Description of the Prior Art
Precipitation and growth of silver halide (AgX) grains for use in photographic processing is generally carried out in a stirred tank reactor with two continuously flowing inlet streams: an aqueous silver nitrate stream, and an aqueous "salt" stream, which is typically a solution of one or more halide salts (for example, NaBr and NaI). In order to obtain the desired size distribution and morphology of AgX grains, it is necessary to control the concentration of excess halide ion during the precipitation and growth of the grains. In a conventional system, each reactant inlet stream is controlled by a delivery system which includes a flow controller that monitors the inlet stream flow rate, and if necessary, adjusts the flow pump to maintain the desired flow rate.
Typically, current practice is to infer the halide ion concentration within the reactor from the voltage measured at an electrochemical cell and silver electrode immersed within the reactor. This inferred halide ion concentration is then used to calculate the required adjustment to the current flow rate of the salt inlet stream. The method used for this calculation is a simple mass-balance relation and the process is repeated with a new voltage reading at regular intervals (e.g., at 5 second intervals). This practice suffers from two major flaws.
First, the voltage signal obtained from the silver electrode is quite noisy due to turbulent concentration variations in the local vicinity of the electrode. Since the variations are a fundamental property of the reactor, this noise cannot be eliminated. The undesired impact on the concentration-control method is that the current method can not distinguish fluctuations due to "noise" (and hence unavoidable) from fluctuations due to genuine shift in the mean halide concentration in the reactor (and hence requiring corrective change in the salt flow rate). This error often results in a rapid and excessive fluctuation in salt-feed flow rate, as the system attempts to control "noise." These excessive salt-feed fluctuations lead to nonuniformities in reactor conditions, which are believed to cause degradation in AgX grain size uniformity and morphology. The flow rate fluctuations also cause excessive wear on the salt feed delivery system.
The second major problem with the present control approach is with the method of calculating the required adjustment of salt feed flow rate[, i.e.,] given a value of the current halide ion concentration in the reactor. Currently, an equation is used that depends on having perfectly accurate values for the concentrations of both the salt and silver feed streams, as well their flow rates. In practice, however, the feed concentrations are only known to within approximately 0.5%, and the flow meter calibrations can be incorrect by a similar amount. These unavoidable errors prevent the current method from producing a correct adjustment for the salt feed flow rate and hence prevent the system from obtaining the desired halide concentration level in the reactor.
The present invention solves both of these current practice flaws.